1. Field of the Invention
The present invention relates to an apparatus for non-invasively measuring the degree of oxygen saturation in blood, and more particularly to an apparatus for non-invasively measuring the oxygen saturation by utilizing the optical absorption and scattered light characteristics of hemoglobin contained in red blood corpuscles.
2. Description of the Prior Art
There is a conventional apparatus for non-invasive measurement of oxygen saturation known as a pulse oximeter. A pulse oximeter works by measuring changes in transmittance accompanying the pulsing of arterial blood, and calculating the oxygen saturation of red blood corpuscle hemoglobin. Focussing also on red blood corpuscles in blood vessels, JP-A-HEI-4-15046 discloses a speckle oximetry method of measuring blood oxygen saturation.
As pulse oximetry measures changes in transmittance accompanying arterial blood pulses, it may be unable to detect the pulse when peripheral blood vessels are contracted, such as when a subject is in shock, for example. In such cases measurement can be difficult or impossible. Measurement also becomes difficult when the pulse is very weak owing to heavy hemorrhaging. Moreover, it is very difficult to measure the degree of oxygen saturation in venous blood.
It was to resolve such problems that the speckle oximetry method was prepared in JP-A-HEI-4-15046. In the speckle oximetry method a plurality of laser beams with different wavelengths are focussed simultaneously on a living subject, the light scattered therefrom is separated into the different wavelengths, and the oxygen saturation of the blood is measured from the intensity fluctuations of each wavelength.
However, there is a limit to the laser beam irradiation intensity that can be used on a living subject, and irradiation by a plurality of high power laser beams is not possible. Moreover, it is desirable to decrease the intensity of the laser beam that is used to avoid low-temperature burns to the living tissue. A low intensity is also desirable from the standpoint of ease of handling. When using a plurality of laser beams simultaneously, even if the intensity of individual beams is low, the effect that the total amount of irradiation involved can have on living tissues becomes a problem. If the output of each laser is decreased further in an attempt to solve the problem, the result is a lower signal-to-noise (S/N) ratio, lowering the accuracy of measurements.
An object of the present invention is to provide an apparatus that irradiates a living subject with coherent light, such as a laser beam, to non-invasively measure the oxygen saturation of hemoglobin in red blood corpuscles, and can readily and accurately measure oxygen saturation whether in arterial blood with a very weak pulse or in a blood flow with no pulse such as venous blood, and which moreover can reduce the amount of laser beam irradiation to which the living body is subjected.